perf: optimize MQTT NatsToMqtt fast path and pre-warm topic cache

Add string.Create fast path in NatsToMqtt for subjects without _DOT_
escape sequences (common case), avoiding StringBuilder allocation.
Pre-warm the topic bytes cache when MQTT subscriptions are added to
eliminate cache miss on first message delivery.

src/NATS.Server/Mqtt/MqttNatsClientAdapter.cs

@@ -83,6 +83,9 @@ public sealed class MqttNatsClientAdapter : INatsClient
     /// </summary>
     public Subscription AddSubscription(string natsSubject, string sid, string? queue = null)
     {
+        // Pre-warm topic bytes cache for this subject to avoid cache miss on first message.
+        MqttTopicMapper.NatsToMqttBytes(natsSubject);
+
         var sub = new Subscription
         {
             Client = this,

src/NATS.Server/Mqtt/MqttTopicMapper.cs

@@ -107,7 +107,18 @@ public static class MqttTopicMapper
         if (natsSubject.Length == 0)
             return string.Empty;
 
-        // First, replace _DOT_ escape sequences back to dots
+        // Fast path: no _DOT_ escape sequences — just char replacement via string.Create
+        // (avoids StringBuilder allocation for the common case).
+        if (!natsSubject.Contains(DotEscape))
+        {
+            return string.Create(natsSubject.Length, natsSubject, static (span, src) =>
+            {
+                for (var i = 0; i < src.Length; i++)
+                    span[i] = src[i] switch { '.' => '/', '*' => '+', '>' => '#', _ => src[i] };
+            });
+        }
+
+        // Slow path: has _DOT_ escape sequences — use StringBuilder
         var working = natsSubject.Replace(DotEscape, "\x00");
 
         var sb = new StringBuilder(working.Length);